FIG. 1 shows an example of an optical transmission module comprising (LD+MPD) that is in actual use at present. This is an LD module of a cylindrical metal package type that incorporates a lens 12, a lens holder 13, an LD 4, an MPD 5 and the like within a cylindrical metal case (not shown). The semiconductor laser (LD) 4 is affixed at an intermediate part of a pole 2 mounted on a disk-like metallic stem 1 that projects lead-pins 7, 8, and 9 downwardly from the bottom thereof, and that has the pole 2 vertically formed on the surface thereof. The monitoring photodiode (MPD) 5 is affixed, directly below the LD 4, to the stem surface via a sub-mount 6. The sub-mounts 3 and 6 of the LD 4 and the MPD 5 are connected to the lead-pins 8 and 9 by bonding-wires 10 and 11, respectively. A lens 12 is disposed directly above the LD 4.
The lens 12 is attached to a cylindrical lens holder 13. After the lens 12 and the LD 4 having been aligned with each other, the lens holder 13 is soldered to the stem 1. The front end 14 of an optical fiber is held by a cylindrical ferrule (not shown), and is inserted into a ferrule holder with a tapered cylinder shape. After being aligned, the ferrule holder is soldered onto the lens holder 13. Furthermore, a bend-limiter (not shown) is inserted into the lens holder 13 in order to inhibit excessive bending of the optical fiber.
The optical transmitter accommodated in such a metal package has various advantages. Since the optical transmitter is sealed by metal and is filled with an inert gas, it is protected from moisture and oxygen, thereby allowing the LD and the MPD not to degradate easily. Also, since the optical transmitter is sealed by a metallic case, the entry and occurrence of noises is prevented. Signal light 15 once travels out in the space, but since it is condensed by the lens, it is efficiently made incident on the optical fiber. It is therefore possible for the optical transmitter to cover signals up to a high frequency region. With these superior properties, the metal package type optical transmission module (LD+MPD) has assumed a dominant position in transmission modules in optical communication.
The optical transmission module of a metal package type shown in FIG. 1 has already established a track record, and is well-known. However, in this optical transmission module, costs for components such as the package and the lens are high, and assembly cost is also high, since time and effort are consumed on the alignment of the center. In addition to the above-described drawback of being high-cost, the optical transmission module also has a large volume, thereby requiring a wide space when mounted on a printed circuit board.
For such a reason, surface-mountable LD modules have been proposed in order to reduce cost. Among various types proposed, for example, as a prior art, “Optical Coupling Characteristics of Resin Mold Type LD Module” by Koji Yoshida, Takeshi Kato, Toshinori Hirata, Fumio Yuki, Kimio Tateno, and Toshio Miura, 1997, The Institute of Electronics, Information and Communication Engineers, General Convention, C-3-68, p. 253, proposes a surface-mountable LD module in which an optical fiber, an LD, and an MPD are arranged in one plane on an Si-substrate. FIG. 2 shows the configuration in outline of this prior art. This module is under development, and not yet in actual use. A V-groove 18 is formed along a central axis line up to a midway of a planar Si-substrate 17. A single mode optical fiber 19 is halfway embedded in the V-groove 18. At positions on an extension line of axis of the optical fiber 19, an LD 20 and an edge illuminated MPD 21 are mounted on the Si-substrate 17.
The edge illuminated MPD 21 is also referred to as a “waveguide type PD”, since the light receiving part 24 thereof is provided along a waveguide. While the MPD 21 is a particular MPD, it has a structure allowing horizontal incidence, so that a core 22 of the optical fiber, a light emitting part 23 of the LD, and a light receiving part 24 of the MPD 21 are arranged in a straight line. The electrode of each of the LD 20 and the MPD 21, and the metallized part on the Si-substrate are interconnected by wire bonding.
The end of the optical fiber 19, the LD 20, and the MPD 21 are covered with a transparent resin 25. The front light of the LD enters the optical fiber 19 and propagates therethrough. The rear light of the LD travels horizontally and directly enters the MPD 21, by which the rear light is detected. In this module, by mounting both the semiconductor laser and monitoring photodiode on the same Si-substrate, it is possible to reduce the assembling process, and simultaneously to reduce its size.
With regard to the function of this edge illuminated MPD, the rear light of the LD is received by the monitoring photodiode, and a semiconductor laser driving circuit, (which is included in another module), is controlled so that the optical output power W becomes constant, and the average value <W> of the transmitting light (front light) of the semiconductor laser is maintained constant. While having a very simplified structure, the edge illuminated MPD is a particular element and is lacking in universality. In addition, the edge illuminated MPD suffers from a drawback of a low incidence efficiency of the LD rear light.
Conventional optical transmitters (LD+MPD) have further problems. An example of a conventional surface-mountable module FIG. 2) has a simple configuration in which an optical fiber, an LD, and an MPD are arranged in a straight line. Since the LD and the MPD are mounted on the same Si-substrate, it appears possible to enable a simplified mounting process. However, the light receiving part (MPD) is of a waveguide type, and the area of the part on which light can be made incident is small. This causes a low coupling efficiency of monitoring light. Also, the alignment requires strict accuracy to the same extent as the case of mounting the semiconductor laser. When light is made incident on the edge face, the light beams would fail to enter the MPD if light beams are vertically deviated at a minimum, because the waveguide has a small thickness. That is, most of the rear light would not enter the MPD. Since the monitoring light is weak, the reliability of the feedback control of the LD is low.
Therefore, in order to further draw on the features of the surface mounting technique, a simpler layout of the monitoring photodiode is desirable. A structure that allows the light amount entering the MPD to increase by simplifying the relationship between the LD and the MPD and thereby facilitating the mounting, is earnestly required. It is a first object of the present invention to propose an optical communication module equipped with an LD module that allows more LD rear light to be sensed and that enables a higher accuracy of LD current control. Moreover, it is a second object to propose an optical communication module equipped with a compact and low-cost LD module.